The present invention relates in general to the design of light emitting diodes (LEDs) and laser diode structures and in particular, to an LED with a multiple lobe optical spectrum and a laser diode having multiple designated wavelengths.
Multiwavelength LEDs and laser diodes are key components for applications in multiplexed fiber optic data links and sensor networks. One example of this application is the standard fiber-optic interface system in future generation aircraft. The use of wavelength division multiplexing (WDM) techniques in the system is primarily due to a large number (e.g. 40-50) of sensors and data channels to be processed. The resolution of typical WDM elements is limited to 5-10 nm at the center wavelength of 800 nm. Therefore, optical sources with a spectral width of 200-500 nm and good coupling efficiency to the optical fiber are required. These dictate the need for specific edge-emitting LED designs for the unconventional spectral output.
Prior art LED and laser structures consist of an active layer sandwiched by a pair of larger bandgap cladding (or confinement) layers. The active layer can be a single layer on the order of 1000 Angstroms (1 Angstrom=10.sup.-8 cm) or a series of alternate thin layers (namely, Quantum Well and Barrier, respectively) on the order of 10s and 100s of Angstroms. The emission wavelength for the single layer case is determined by the bandgap energy of the semiconductor.
For the Quantum Well case, the emission wavelength is governed by the transition between the quantized energy levels in both the by the transition between the quantized energy levels in both the conduction and valence bands, which are a function of the Quantum Well thickness as shown in FIG. 1. The wavelengths of Quantum Well lasers and LEDs that can be achieved are shorter than in conventional devices. In addition, by virtue of the quatum size effect, the structures also show a lower "transparency" carrier (current) density, lower optical loss, and higher Quantum efficiency.
In U.S. Pat. No. 3,676,795, entitled "Multiple frequency Laser Apparatus", the inventor discloses an apparatus and method of obtaining multifrequency laser action out of a semi-conductor. This prior art invention, however, requires externally applied stress such as shear or uniaxial stress at variable degrees which is a relatively inefficient technique of achieving multiple wavelengths.
In U.S. Pat. No. 4,637,122, entitled, "Integrated Quantum Well Lasers having Uniform Thickness Lasing Regions for Wavelength Multiplexing", Carney et al. disclose an integrated quantum well laser structure which provides a plurality of light beams each having a different wavelength for use in wavelength division multiplexing. Unfortunately, the range of wavelengths emitted by the laser is limited to the cavity loss in the Quantum Well.
In U.S. Pat. No. 4,747,110, "Semiconductor Laser Device capable of emitting laser beams of different Wavelengths", Takahashi et al. disclose a semiconductor laser device in which the wavelength range is limited to the variation of the Quantum Well thickness. The wavelength variation, in effect, is limited to the rate of growth on crystallographic orientation.
Most conventional LEDs and lasers are designed for a narrow spectral width. Prior art devices teach that the conventional way of achieving multiwavelength LED or laser diode structure comprises an array of conventional LEDs or lasers with various wavelengths which requires large number of fibers and connectors. This results in inefficient or low optical power and reliability for the systems. A new design is therefore needed to increase the efficiency and reliability of multiwavelength LEDs/lasers to be used in various applications such as in WDM techniques.